Oxides of nitrogen (NO.sub.x) emitted from industrial processes can be controlled under most operating water absorber. However, occasionally, NO.sub.x evolves from industrial processes at extremely high rates for very short periods, which overwhelms the standard equipment, producing observable emissions for a short duration.
In batch metal dissolution processes using hot nitric acid, conventional methods for treatment and control of NO.sub.x emissions consist of: (1) passing the reactor effluent gases through a water absorber, (2) treating the reactor effluent gases with a caustic scrubber and/or (3) adding prilled urea to the reactor, which slows the reaction rate and associated NO.sub.x generation. Such treatment methods are used in addition to optimizing reactor conditions for dissolving metal to prevent or minimize a generation of NO.sub.x.
It is inefficient to operate conventional treatment methods on a scale which accommodates for the peak evolution of NO.sub.x, particularly in batch metal dissolution processes where, during much of the cycle, NO.sub.x evolution is moderate.
The use of chemical treatment agents in absorber columns to accommodate for the peak evolution of NO.sub.x from batch metal dissolution processes can be effective and will not affect the reaction rate. However, such methods are inefficient and unnecessarily costly if the treatment agent is fed continuously into the absorber column since scrubbing with water alone will remove substantially all NO.sub.x from emissions during most of the batch process. It is desirable to add chemical treatment agents only as needed, typically just prior to increases in NO.sub.x emissions.
NO.sub.x analyzers are available which monitor the concentration of the effluent components at the exhaust flue. These analyzers can detect increases in NO.sub.x emissions. However, these NO.sub.x analyzers are expensive and require significant maintenance in that they should be recalibrated every one to three days. Furthermore, the time to respond to the analyzer readings is very short in that the effluent which is monitored is almost outside of the flue. Observable NO.sub.x plumes can still be a problem and to avoid significant emissions of NO.sub.x, the system must respond rapidly and excessively to any change in NO.sub.x concentrations which are detected. It is desirable to provide a method for removing the oxides of nitrogen (NO.sub.x) from a gaseous effluent utilizing a control means which is simple and economical to operate and which provides adequate time to respond to increases in NO.sub.x emissions so as to avoid observable plumes of NO.sub.x.